Learn Swedish: Vilket tåg är rätt?

Vilket means "which" and it changes to match the gender of the noun it belongs to. Because tåg (train) is an ett-word (ett tåg), we must use the matching form vilket. If we were asking about an en-word like bil (car), we would say vilken bil.

Why is it just tåg and not the definite form tåget?

In Swedish, the word for "which" (vilken, vilket, or vilka) is always followed by the indefinite (basic) form of the noun. It works exactly like English! So you say vilket tåg (which train) rather than vilket tåget.

Does rätt mean "right" as in "correct" or "right" as in the direction?

In this sentence, rätt means "correct". If you were asking about the physical direction "right" (as the opposite of left), you would use the word höger.

How would I ask "Which trains are right?" if I'm asking about more than one train?

You would use the plural form for "which", which is vilka. The sentence becomes Vilka tåg är rätt?. Notice that tåg looks exactly the same here because ett-words that end in a consonant do not change their spelling in the plural!